This invention relates to controlling fluid flow over a surface, especially transonic flow, and has particular but not exclusive application to aircraft traveling at high Mach numbers, a situation where flow separation can occur.
Flow accelerates as it passes over the forward increasing cross section of an obstruction. As the local flow velocity increases the local static pressure decreases. When the flow tries to close in behind the obstruction it must both decelerate and increase in pressure. If the amount of local deceleration and pressure increases are too great the flow separates from the body surface. This phenomenon normally occurs in subsonic flow because of viscous losses in the foot of the boundary layer which prevent the necessary pressure recovery. In transonic flow (subsonic flow having an embedded region of supersonic flow), separation normally occurs because of the steep pressure recovery required by the recompression shock added to the gradual pressure recovery required by the surface curvature. In general, the higher the freestream (incident) velocity and the more bluff the obstruction, the greater the increase in boundary layer edge flow velocity and the more difficult it is for flow to remain attached past that part of the obstruction having the greatest cross-section transverse to the flow direction.
The undesirable effects of flow separation range from annoying to catastrophic. From a simple fluid mechanical standpoint, separation increases body drag, unsteady buffet pressure loads on the body, and loss of lift. From an aero-optical standpoint, separation causes unsteady shear layers through which it is difficult to propagate optical radiation without unacceptably large and nonuniform losses in intensity. These negative effects increase with the freestream Mach number, especially in the transonic range with the creation of additional vorticity downstream of the recompression shock.
It has been known since the 1920's that removal of the low kinetic energy foot of the boundary layer (the part closest the surface) by distributed suction through holes in the surface under the boundary layer will prevent separation in all subsonic flow. Recent testing has shown that this technique is ineffective in transonic flow.
In weak transonic flow the most commonly used technique for at least delaying separation, that is, causing separation to occur farther downstream around an obstruction such as a bluff body or airfoil, is to use short vertical fins attached to the surface which forces the mixing of the higher and lower energy regions of the boundary layer and imparts energy to the fluid near the surface. This technique is most effective where the boundary layer is thin. A variation in this technique involves the use of spaced jets which are injected across the incident stream. Both of these techniques have been referred to as vortex generators because of the relatively large scale vortex-like mixing they induce.
One of the most effective techniques for delaying separation in moderately transonic flow with thick boundary layer is to use streamwise slot blowing oriented as nearly tangentially to the downstresam surface as possible. The major drawbacks in this technique are that it causes boundary layer thickening and that it has a limited range of influence.
A final flow separation suppression technique involves rotating the surface in the direction of flow to weaken the velocity gradient through the boundary layer. For practical reasons, this technique has been primarily of academic interest.
All of the techniques described above fail to prevent shock-induced separation in moderately strong transonic flow.
Another technique known in the prior art for controlling subsonic flow over a surface involves generating a single, long vortex in the surface which rotates about an axis transverse to the flow. For example, U.S. Pat. No. 2,841,182 to Scala shows the use, in a diffuser, of a stepped-slot for retaining a large driven vortex. The vortex is sustained by withdrawing low-energy fluid from the center of the vortex using a perforated tube at the center of the slot.
It is also known to use a single, long spanwise vortex on an airfoil to increase its efficiency. For examle, French Pat. No. 825,134 to Riabouchinsky shows the use of centrifugal pumps located at either end of a long spanwise slot on an airfoil which pump air from the center of the slot and so sustain a long spanwise vortex within it.